Spatial variability

A snow-covered landscape is a tranquil sight. But what does the snowpack conceal? The safety authorities and backcountry tourers would like to know as much as possible about the snowpack's properties in avalanche starting zones and on ski slopes. Has the recent snowfall covered a dangerous weak layer, or has the wind created a slab somewhere?

A snow-covered landscape is a tranquil sight. But what does the snowpack conceal? Safety authorities as well as backcountry recreationists would like to know as much as possible about the snowpack's properties in avalanche starting zones and on ski slopes. Has the recent snowfall covered a critical weak layer, or has the wind created a slab somewhere?

The release of a slab avalanche is preceded by fractures in the snowpack. Where the initial crack occurs and the extent to which it propagates depend on the properties of both the slab and the weak layer underneath it – properties that may vary over a given area. For avalanche forecasting not only the snowpack's vertical layering, but also its spatial variability is therefore relevant. The scientists in the avalanche formation group investigate the causes, characteristics and consequences of snowpack variability (Fig. 1).

Figure 1: Field measurements are performed according to a predetermined pattern in a basin with a variety of slope aspects.

In order to record spatial variations in snow properties, it is essential that a large number of measurements can be made in a reasonably short time at many locations over the area of interest. Remote sensing methods do allow this, but little reliable information has been obtained from the snowpack thus far, in particular concerning its stability. Only stability tests can deliver the information required to examine the likelihood of a failure at a layer boundary. Such tests allow the stability to be assessed at a specific point in the terrain, but they are time-consuming. With the help of the snow micro-penetrometer (SMP) (Fig. 2) developed at the SLF, in contrast, a variety of snowpack properties can be measured in just a few seconds. Recent research has now, for the first time, identified a relationship between the penetrometer signal and the stability of the snowpack. By entering the data captured by the micro-penetrometer into fracture models, scientists can simulate the outcome of a stability test. This approach avoids digging in the snowpack and, with the aid of the penetrometer, allows collecting the relevant information at many locations in e.g. a catchment within a short time so that the stability and its distribution can be assessed. A picture of the snowpack stability in the field (Fig. 3) is produced by way of interpolation between the measuring points.

Figure 2: The snow penetrometer was developed by the SLF. Scientists use it to measure the resistance encountered by the tip of the instrument when it penetrates the snowpack. It allows snowpack properties, such as its hardness, to be determined in just a few seconds.

But how does the variability of the snowpack come about? Wind is certainly a key influencing factor; its interaction with the topography gives rise to different quantities of snow being deposited. The structure of the snowpack is to a large extent not coincidental, but governed by the interaction of weather and terrain. SLF scientists are currently studying the processes that cause the spatial variability. They are comparing measured variability with weather conditions simulated with small-scale numerical models. Eventually, an understanding of this relationship will allow the variability to be forecast in future.

Figure 3: The interpolation of snowpack characteristics, such as critical crack length, produces a two-dimensional image of the snowpack's stability. The image shown here covers an area of around 0.1 km2 in a small high valley.

Contact

• Benjamin Reuter